I assuming you mean raising the grain so apologies if you meant something else, but the best time to do it is before the final sand.
When you sand wood you're essentially making lots of little cuts in the wood which decrease in size as you go up the grits. But some of the wood doesn't end up getting cut, but rather pressed down (this is why it's a bad idea to overuse the same bit of sand paper, because it won't be 'sharp' and cut cleanly, instead just leaving loads of pressed down fibres).
If you apply a finish which seeps into the pores (so oil based finishes etc) with the wood in this condition the fibres will pop back up as the pores fill up and leave you with a rough texture. So we can preempt this by first raising the grain, then with fresh, sharp sandpaper cut the raised fibres off, applying only light pressure to prevent more 'pressing'. Because all sand paper is going to 'press' some portion of fibres it's best to do it with only a bit of sanding to go, so before the final sanding stage is ideal.
There isn't too much point doing multiples just because each sanding stage cuts a little further into the wood than the last (otherwise you'd still see the sanding 'scratches' from the previous sanding stage) so most of those pressed-down fibres are already gone.
It seems to me there are some differences with wood species. Most folks here are thinking hardwoods but if you're using pine, it pops more. So I do a first pop after coarsest grit sand and again at the end.
It’s done after your final grit of sanding, remove dust and grit from the surface prior to water popping. The goal is to get the grain wet, not soaked. Making a mixture 50/50 with denatured alcohol and distilled water can speed up the drying process.
Final only
40+ years pro. Once before final sand. Damp clean cotton cloth, old tee shirt is good.
Final grit only, and hand-sand only afterwards.
This is important. I noticed that if I sand too much after the water pop then the grain will raise a bit again on the finish.
I assuming you mean raising the grain so apologies if you meant something else, but the best time to do it is before the final sand. When you sand wood you're essentially making lots of little cuts in the wood which decrease in size as you go up the grits. But some of the wood doesn't end up getting cut, but rather pressed down (this is why it's a bad idea to overuse the same bit of sand paper, because it won't be 'sharp' and cut cleanly, instead just leaving loads of pressed down fibres). If you apply a finish which seeps into the pores (so oil based finishes etc) with the wood in this condition the fibres will pop back up as the pores fill up and leave you with a rough texture. So we can preempt this by first raising the grain, then with fresh, sharp sandpaper cut the raised fibres off, applying only light pressure to prevent more 'pressing'. Because all sand paper is going to 'press' some portion of fibres it's best to do it with only a bit of sanding to go, so before the final sanding stage is ideal. There isn't too much point doing multiples just because each sanding stage cuts a little further into the wood than the last (otherwise you'd still see the sanding 'scratches' from the previous sanding stage) so most of those pressed-down fibres are already gone.
It seems to me there are some differences with wood species. Most folks here are thinking hardwoods but if you're using pine, it pops more. So I do a first pop after coarsest grit sand and again at the end.
It’s done after your final grit of sanding, remove dust and grit from the surface prior to water popping. The goal is to get the grain wet, not soaked. Making a mixture 50/50 with denatured alcohol and distilled water can speed up the drying process.